Why do I get these dead spots?

No guessing allowed. You must learn to think like the press. Well, almost. You must understand how the press was intended to work when it was built. Then you must help it to do its job.
You test, evaluate, and make one change at a time. You may rule out several things until you come to the solution.
You have some information. You can rule out packing and makeready as you have plenty of punch into the paper.
You are not getting sufficient ink on the light spots and it is in a line from top to bottom. That is good information. All you have to do is solve the problem.
One or more of the rollers is apparently not picking up sufficient ink on that path. Observe the rollers carefully. Are you using all the rollers?
Note where the light streak is with relationship to the rollers. Turn the chase upside down and make a print. Is the light spot in the same location with relationship to the rollers? Move the form in the chase just a bit left or right. Make another print. Is the light streak in the same location with respect to the rollers? By now you should have some pretty good information. Have you come to the conclusion that it is an inking problem with the rollers?
Clean and remove the rollers. Roll them on a very flat surface like a glass topped table. Put a flashlight behind and look for light leaking from under.
I am not there to touch the patient, but I think you will find one or more flat spots.

I would also suggest making sure that your bearers and trucks are free of any oil. I have seen this effect as a result of rollers sliding on the forme. What kind of trucks do you have?

DGM

Another factor to check might be the presence of a ding or scratch or tenacious bit of dirt on either roller truck, which could cause it to hop over a place on the plate, but which might shift relative position during the run — sometimes the rollers spin a small amount at the bottom of their travel and that would move the interfering flaw and rotate the roller slightly, which would result in the under-inked area shifting position.

Bob

Thanks everyone!

Inky - thank you for the reply. I feel like I’m constantly having to solve problems and find solutions on this press, but it’s always worth it :) I did check the rollers, no flat spots. I’ll have to turn the chase another day though and check that.

DGM - I have Delrin trucks that I bought at the same time as my rollers around 3-6 months ago. I’m not a daily printer so everything is still pretty new and in good condition. It doesn’t seem to have any oil on it, but I could try giving my trucks a better clean on my next run.

I have a bit of a gut feeling it’s got something to do with my trucks, so I’ll keep an eye on them next time.

I don’t want to insult anyone’s intelligence, but just to make sure we cover everything, I’m assuming your roller cores (shafts) do have places on each end just past the roller material, where the metal looks like it is “pinched up” a little, and these “pinched up” places on the cores fit into keyways or slots in the trucks. This insures that the trucks and rollers always turn together. Your rollers and trucks do have these, right?

I ask this because when I got new rollers and plastic trucks for my Golding Pearl #3, these were missing and I had to fabricate something to perform that function.

Geoffrey, you have just inadvertently solved a minor niggle that I’d had for a while with that little reminder - thank you so, so much!

You do not say whether the uninked strip runs across the page or down.

If across the page, and the rollers are set a bit too low, they could be hitting the leading edge of the page and ‘jumping’, thus failing to ink a small strip of the page.

I assume you have checked the roller settings with a roller gauge?

I was thinking these might be Delrin trucks. I have them on a C&P Pilot and have sometimes found them to slide because they are so smooth. A rosin bag is typically the appropriate way to get a bit more traction, but in a pinch, I used the green side of a kitchen sponge to take the shine off of them and they behaved better.

DGM

I have run into spots like that -parallel to the rollers on an 8x12. I have solved it by lowering the rollers very, very slightly on one side. Raise the rollers back up and it appears again. It could be more than roller height in play here because I have run into this problem with specific plates (photopolymer) and not others under the same conditions. Constantly having to solve problems and find solutions on the press is what it’s all about.

PtolemyPhil, I’m glad my comment was helpful to you. Thanks for the feedback…….Geoff

Update: I checked every comment and took everyone’s advice. Thanks again to everyone, you’re all awesome.

Dwallen - your advice nailed it. I slightly lowered one side of my rollers and haven’t seen a dead spot since. Thank you!